Documentation update only.

Thanks to @ryan-senn for his fixes for two documentation types.

I have also updated the documentation for Period and Duration to try and make it clearer, fixing a problem in an example cause in Duration that has caused confusion.

With this work I have realised that when i used the concepts of Period and Duration from nodatime that I managed to swap them around in elm-date-extra. My apologies if I have confused anyone it was not intentional.

I have not yet decided if I should swap the the Period and Duration module functionality over so they align with nodatime as yet.